[1] OFDM and Cell Search Processing:
The OFDM modulation scheme uses a guard interval (GI) which is a copy of a part of an effective symbol (effective 20 data) and added to the effective symbol for the purpose of reducing deterioration of characteristics due to delay waves. Since the length of a guard interval added is determined based on an expanse of delay in a propagation path, an embodiment in which multiple guard interval 25 lengths are switched in an operation is proposed.
As an example of such an embodiment, there is a system in which the number of symbols transmitted by subframes of the same length is changed, and the guard interval length is adjusted. In such a system, in a cell with a large radius (hereinafter will be called a large cell) which evolves in the suburbs where few objects executing a shielding effect are present, a subframe format (long GI subframe) having a long guard interval length Ngi_s is used as shown in (2) of FIG. 23, and contrarily, in a cell with a small radius (hereinafter will be called a small cell) which evolves in urban areas where a lot of objects executing a shielding effects are present, a subframe format (short GI subframe) having a short guard interval length Ngi_s is used as shown in (1) of FIG. 23 (for example, see the following non-patent document 6). In this instance, in this FIG. 23, N0 indicates the length of an effective symbol, and one OFDM symbol is formed by one guard interval and one effective symbol.
In cellular systems, it is necessary for mobile stations to perform cell search processing which is an operation of searching a cell with which the mobile station is to establish a radio link. Hereinafter, a description will be made of an example of cell search processing in a case where subframe formats of multiple guard interval lengths exist in a mixed manner thereof.
FIG. 24 shows a construction of a base station transmitter apparatus. The base station transmitter apparatus of FIG. 24 includes, for example: a channel multiplexer 101; a serial/parallel converter 102; an inverse fast Fourier transformer (IFFT) 103; a guard interval inserter 104; a guard interval length controller 105: a radio unit 106; and a transmitter antenna 107. After the channel multiplexer time-division multiplexes a signal (symbol) of a data channel, a signal (symbol) of a pilot channel, a signal (symbol) of a synchronization channel (SCH), and etc., the serial/parallel converter 102 performs serial/parallel conversion of the time-division multiplexed signals to map the converted signals to each subcarrier. The inverse fast Fourier transformer (IFFT) 103 performs IFFT processing, the time-division multiplexed signal thereby being converted into a time domain signal. In this instance, in the following description, signals (symbols) of the above mentioned various channels sometimes will be simply called “so-and-so channels” in a shortened manner. In addition, a pilot channel signal will also be simply called “a pilot signal” or “a pilot”.
The time domain signal is input to the guard interval inserter 104, and a guard interval of a length (in FIG. 23, Ngi_s or Ngi_l) determined by the guard interval length controller 105 is inserted to the time domain signal by the guard interval inserter 104. The resultantly obtained signal is then transmitted toward a mobile station as a downlink radio signal by way of the radio unit 106 and the transmitter antenna 107.
FIG. 25 illustrates the construction (format) of a subframe containing seven OFDM symbols per subframe of the above mentioned radio signal. As shown in FIG. 25, the subframe has a construction such that various channels (OFDM symbols) are multiplexed in the two-dimensional direction with time and frequency. That is, a pilot channel shown by the diagonally shaded part 111, a synchronization channel (SCH) indicated by the diagonally shaded part 112, and a data channel indicated by the reference character 113 from which these diagonally shaded parts 111 and 112 are withdrawn, are time-division multiplexed in each subcarrier (frequency) (each row of FIG. 25), a subframe thereby being constructed.
Here, the synchronization channel (SHC) has a common pattern in all the cells, and is time-division multiplexed to the end of a subframe. The pilot channel has a scramble code which is information unique to a cell, and is time-division multiplexed to the head of a subframe. The mobile station is capable of identifying existing cells by means of using such scramble codes. In this instance, the following non-patent documents 1 and 2 also describe a downlink channel construction and cell search processing on the OFDM base.
Subsequently, a cell search processing sequence is shown in FIG. 26. First of all, on the first stage, correlation with the replica of a time signal of the synchronization channel (SCH) which has already been known is detected, and for example, timing indicating the maximum correlation value is assumed to be subframe timing (step S100).
On the second stage, fast Fourier transform (FFT) processing is performed with the subframe timing detected on the first stage (that is, the detected subframe becomes FFT timing) to generate frequency domain signals, and extracts the above mentioned pilot channel from the generated signals. Then, correlation between the extracted pilot channel and the candidate scramble codes (pilot replicas), and for example, a candidate scramble code showing the maximum value is determined to be a detected scramble code (step S200).
In addition, as an example of a previous cell search processing, there is another technique proposed in the following non-patent document 3. This technique is the three-stage fast cell search method using a pilot channel in downlink broadband OFCDM. The technique groups scramble codes beforehand, and detects a scramble code group before scramble code identifying processing. This makes it possible to narrow scramble codes at the time of detecting a scramble code, so that the speed of cell search processing is enhanced.
[2] MBMS:
In 3GPP (3rd Generation Partnership Project), an investigation of a specification of a communications service of a multimedia and broadcast/multicast type (MEMS: Multimedia Broadcast/Multicast Service) has been progressed for standardization of next generation mobile telephone communications services. For example, the following non-patent document 4 proposes the following: the above mentioned long GI subframe is used in MBMS; the short GI subframe is used in unicast communications; a long GI frame in which MBMS data is multiplexed and a short GI subframe is time-division multiplexed (TDM); and MEMS data is frequency-division multiplexed (FDM) in a time-division multiplexed long GI subframe. In this instance, the use of a long GI subframe for multicast communications is also described in the following non-patent document 6.
Further, the following non-patent document 5 proposes a pilot insertion methods as a pilot signal insertion method in MEMS, in which pilot signals are inserted into a narrow time domain in a concentrated manner with an attention paid to Micro Sleep Mode in unicast communications while another pilot insertion method different from that used at the time of unicast communications are used because of the absence of a need for taking a micro sleep mode into consideration. This is considered to be because of the fact that the optimal pilot insertion method is different between unicast communications and multicast communications.    Non-patent Document 1: 3GPP R1-050707, “Physical Channels and Multiplexing in Evolved UTRA Donwlink”; NTT DoCoMo, NEC, SHARP; Aug. 29, 2005    Non-patent Document 2: 3GPP R1-051549, “Cell Search procedure for initial synchronization and neighbour cell identification”; Nokia; Nov. 7, 2005    Non-patent Document 3: Tanno, Arata, Higuchi, and Sawabashi; “The Three-stage Fast Cell Search Method Using Pilot Channel in Downlink Broadband OFCDM”; Technical Report of IEICE, RCS2002-40, CQ2002-40 (2002-04), pp. 135-140    Non-patent Document 4: 3GPP TSG RAN WGI Meeting #43 (R1-051431), “Multiplexing of Multicast/Broadcast and Unicast Services”, Huawei, Nov. 7, 2005    Non-patent Document 5: 3 GPP TSG-RAN WG1 #43 (R1-051490) “On Pilot Structure for OFDM based E-Utra Downlink Multicast”, QUALCOMM Europe, Nov. 7, 2005    Non-patent Document 6: 3GPP TR 25.814 V0.5.0 (2005-11)